The invention relates to implantable prostheses and methods for forming the same. More particularly, the invention relates to implantable prostheses having metallic porous textured surfaces applied during casting, and casting methods for forming the same.
Implantable bone prostheses are often made by an investment casting process. Investment casting first requires the manufacture of a solid model of the article to be cast. The solid model can be made from a meltable wax or from other heat disposable materials by several known techniques. Once the solid model is made, one or more of the solid models are fixed to a riser, such as a wax tree, and the entire assembly is then encased in a refractory binder material. This done by successive treatments of the assembly with a ceramic slurry coating, following by drying the coating between treatments. After final drying, the resulting investment assembly is heated to a temperature sufficient to melt and extract the casting wax or other heat disposable material from within the shell. Thereafter, the shell may be sintered or fired at a higher temperature to strengthen the shell and to burn off any residue. Molten metal is then poured into the investment assembly to fill the cavities once occupied by the solid models.
An important consideration in the design and manufacture of virtually any implantable bone prosthesis is that the prosthesis have adequate fixation when implanted within the body. Early designs of implantable articles relied upon the use of cements such as polymethylmethacrylate to anchor the implant. The use of such cements can have some advantages, such as providing an immediate and secure fixation that does not develop free play. However, the current trend is to use these cements to a lesser extent because of their tendency to lose adhesive properties over time and the possibility that the cements will contribute to wear debris within a joint.
Recently, implantable bone prostheses have been designed such that they encourage the growth of hard tissue (i.e., bone) around the implant. Bone attachment usually occurs, and bone growth is promoted, where the surface of an implantable bone prosthesis is irregular, textured, and/or porous. The interaction of newly formed hard tissue in and around the porous textured surface of the implantable bone prosthesis has been found to provide good fixation of the prosthesis within the body. A greater degree of bone fixation can usually be achieved where bone engaging surfaces of an implantable bone prosthesis are more porous or irregular.
Porous or irregular surfaces can be provided in implantable articles by a variety of techniques. In some instances an irregular surface pattern or surface porosity is formed in an implantable bone prosthesis by post-casting techniques such as embossing, chemical etching, milling or machining. One drawback to using such techniques to provide irregular bone in-growth surfaces in implantable bone prostheses is the significant amount of post-processing time required. Such post-processing operations lead to delays in obtaining the finished product and also significantly increase the cost of manufacturing the device. These post-processing operations can also diminish the mechanical properties of the device.
Textured surfaces are also applied to implantable bone prostheses by joining a plurality of beads to an exterior surface of the prosthesis to provide separate porous surfaces or pore-forming surfaces. Beads can be joined to or formed on implantable bone prostheses by sintering small metal particles or powders to surfaces of the prostheses in desired patterns. Wire-based pads or grids can also be fused to implantable bone prostheses to provide a texture or surface relief features. A drawback of such techniques is that the sintering step required to fuse such materials to bone prostheses is a high-temperature post-processing step that can impart mechanical weaknesses to the prosthesis, distort the dimensions of the prosthesis, and/or alter the properties of the materials from which the prosthesis is made.
Accordingly, there is a need for implantable prostheses with porous surface regions that enhance the fixation mechanics of the implantable prostheses to hard tissue within the body and maintain the mechanical properties of the prostheses. There is also a need for improved methods of forming prostheses with porous surface regions.
It is thus an object of the invention to provide improved techniques for manufacturing bone prostheses having porous surfaces while preserving the mechanical properties of the prostheses. A further object is to provide methods of achieving porous surface regions in cast bone prostheses during casting operations. Another object is to provide bone prostheses having improved fixation mechanics. These and other objects will be apparent from the description that follows.